FS7: 5 reasons the URSA Mini is better than the Sony FS7

[Ben Scott]

Leaks/incorrect/Varying levels of QC in sealing of the mounting of the sensor could lead to the variances in strength of cast people are seeing. The vignette is more likely simply more noticeable for being magenta.

If your answer to a camera with a native iso of 800 having an IR contamination issue is to put an IRND on it you are already losing a minimum of a stop and who can say if that is enough. You're already losing a fair bit of light in the vignette as it is. So you could now be talking about an ISO200 camera.

There is a reason why more mature camera makers maintain an OLPF (with IR cut) as part of the optical path.

 

[Nirv]

Leaks/incorrect/Varying levels of QC in sealing of the mounting of the sensor could lead to the variances in strength of cast people are seeing. The vignette is more likely simply more noticeable for being magenta.

If your answer to a camera with a native iso of 800 having an IR contamination issue is to put an IRND on it you are already losing a minimum of a stop and who can say if that is enough. You're already losing a fair bit of light in the vignette as it is. So you could now be talking about an ISO200 camera.

There is a reason why more mature camera makers maintain an OLPF (with IR cut) as part of the optical path.

Clear UV/IR Cut filters are available from all good stockists and bad ones too.

 

[paulcurtis]

Leaks/incorrect/Varying levels of QC in sealing of the mounting of the sensor could lead to the variances in strength of cast people are seeing. The vignette is more likely simply more noticeable for being magenta.

If your answer to a camera with a native iso of 800 having an IR contamination issue is to put an IRND on it you are already losing a minimum of a stop and who can say if that is enough. You're already losing a fair bit of light in the vignette as it is. So you could now be talking about an ISO200 camera.

There is a reason why more mature camera makers maintain an OLPF (with IR cut) as part of the optical path.

I believe that 800 ISO base is pushed. Apparently there's a value in the DNGs which shows the gain required and it seems that ISO800 is pushed 2 or 3 stops. That puts the actual native ISO of the sensor more like 100/200ISO. This was off the BMCuser forums, take with suitable pinch of your favourite condiment.

(FWIW the RAW in the FS700 is also pushed in the same way, but for ISO2000 it's more like ISO400/500)

cheers
Paul

 

[Ben Scott]

Clear UV/IR Cut filters are available from all good stockists and bad ones too.

And yet not from Blackmagic by default....

 

[starcentral]

What sort of build issue would cause such a magenta vignette?

I think some of this is due to a phoenomenon called "sensor shading". Copying text from some online resources, it is a technical term that describes light fall-off observed towards the edges of an image. Specifically with digital cameras, "shading" describes the result of various physical phenomena such as lens vignetting and the angular dependence of quantum efficiency due to the use of micro lenses.

Well to be honest, I typed it, but have no real idea what this means. But it all reality it sounds like a lot of physics. And it sounds like something physical is causing it. Maybe the micro lenses on the sensor in combination with the sensor compartment, some light bouncing around and lack of IR and by some stroke of luck - magenta tinted corners.

Is holding a bit of paper over the bare lens mount a usual test? I've never heard of it before.

The "take your lens off white paper test" probably isn't a good test (even though my Sony seemed to pass with flying colors) because from what I've read about sensor shading, when you use lenses at higher apertures the effect is minimized. So using no lens, or shooting wide open can cause it to appear more on some sensors / systems. Someone got the idea to do it with the URSA, and now everyone is doing it to test their camera. Is it a good test? Who knows.

JB posted a white paper test on Alexa XT the other day and it also had some very very light magenta on it I believe. So we can assume it's sort of normalish but when I watch through most new USRA 4.6K mini footage I can spot the magenta so long as the scene has 30-40% IRE or higher image brightness on one or all of the four corners.

 

[Ben Scott]

I believe that 800 ISO base is pushed. Apparently there's a value in the DNGs which shows the gain required and it seems that ISO800 is pushed 2 or 3 stops. That puts the actual native ISO of the sensor more like 100/200ISO. This was off the BMCuser forums, take with suitable pinch of your favourite condiment.

(FWIW the RAW in the FS700 is also pushed in the same way, but for ISO2000 it's more like ISO400/500)

cheers
Paul

I was being generous

For the record I think the FS7 EI2000 is closer to 1250 as well.

 

[morgan_moore]

Sensor shading is light getting in to the 'well' of the sensor or not.

Think of a pixel as a U shape with the bit that matters (the reader) at the bottom.

Now if you fire your light straight down on to the U all the light hits the bottom.. but if your light is coming from an angle then the bottom of the U is shaded by the sides.

It is known that lenses whose rear element is close to the sensor induce more problems as the light tends to be coming from an angle

S

 

[starcentral]

Sam, yes, somewhat dependent on the angle of entrance to the photosite.

Sensor based vignetting is a real problem for cameras with a large sensor and short flange distance (distance from mount to sensor). This is simply due to the oblique angles of incidence of light hitting the edges of the sensor, which isn't really flat like a piece of film, but contains a stack of objects starting with UV/IR/AA filters, followed by microlenses, colour filters, then many layers of silicon with the photosensitive area at the bottom of a deep well!

Several things can be done to address the problem. Minimising the thickness of the filter stack certainly helps, though Leica attempted this with the M8 but made the IR filter too thin making the camera very sensitive to IR light, causing strange colours in the image (they even had to give out free IR screw on filters to angry customers).

Doing away with the anti-aliasing filter is another option that Leica and Sony (with the A7r) have pursued. Another option is offset microlenses (Leica M9) or elliptical microlenses (Leica M240), both of which attempt to aim the oblique light rays more effectively toward the photosites. The problem with this is that it compromises performance with longer focal length lenses.

The best solution is to get the photosensitive area of the silicon as close to the top as possible. One way is to thin the back of the wafer until it becomes transparent and then flip the sensor around so the readout circuitry is behind the photosite. These so called "back side illuminated" sensors are in existence, but only very recently in anything but very small cell phone cameras.

Finally organic sensors have been touted as the solution to ray angle issues, these have an organic layer that is photosensitive at the very top with circuity underneath. They are a way of production, but promise to eventually deliver the utopia of tiny wide angle lenses that ought to have been the result of doing away with the DSLR mirror!

http://photo.stackexchange.com/questions/43451/how-does-a-sensor-cause-vignetting

 

[Nirv]

I think some of this is due to a phoenomenon called "sensor shading". Copying text from some online resources, it is a technical term that describes light fall-off observed towards the edges of an image. Specifically with digital cameras, "shading" describes the result of various physical phenomena such as lens vignetting and the angular dependence of quantum efficiency due to the use of micro lenses.

Well to be honest, I typed it, but have no real idea what this means. But it all reality it sounds like a lot of physics. And it sounds like something physical is causing it. Maybe the micro lenses on the sensor in combination with the sensor compartment, some light bouncing around and lack of IR and by some stroke of luck - magenta tinted corners.




The "take your lens off white paper test" probably isn't a good test (even though my Sony seemed to pass with flying colors) because from what I've read about sensor shading, when you use lenses at higher apertures the effect is minimized. So using no lens, or shooting wide open can cause it to appear more on some sensors / systems. Someone got the idea to do it with the URSA, and now everyone is doing it to test their camera. Is it a good test? Who knows.

JB posted a white paper test on Alexa XT the other day and it also had some very very light magenta on it I believe. So we can assume it's sort of normalish but when I watch through most new USRA 4.6K mini footage I can spot the magenta so long as the scene has 30-40% IRE or higher image brightness on one or all of the four corners.

Interesting Dennis.

 

[paulcurtis]

It could well be the angle of light hitting the micro lenses and perhaps the paper test isn't ideal.

As i touched on above the lens mount for the ursa is EF, which is about 44mm from the sensor, compared to (18?) for emount. The angle of light from the rear of a lens on emount is much greater, especially with lens elements that are back from the mount too (typically wide angle). This is why we get shading on Leica/EMount with some designs of lens.

With a lens further away from the sensor the light is more telecentric, it's comes out straighter which should help with this.

But with the paper test the light is everywhere and it's not focused, so in fact perhaps the paper test is worst case, with light bouncing everywhere.

Maybe the fact that different people have seen this by different amounts is simply down to different lenses being used. Even for the same focal length the design of the lens itself can make a difference.

Could be a non-issue (again)

cheers
Paul

 

[Ben Scott]

I blame the illuminati.

 

[starcentral]

Yes Paul, I was thinking that EF might actually be the worst lens mount for the URSA mini design due to distance and angle and that perhaps PL lenses (at 52mm) might give it that extra amount to eliminate the issue.

The PL's are just starting to ship so hopefully shooters start to compare the issue from lens to lens, including any lens mount adapters being used, and EF vs PL.

I could go for a PL mount version of the mini 4.6k if this is the case, as I have a full set of them.

 

[paulcurtis]

Yes Paul, I was thinking that EF might actually be the worst lens mount for the URSA mini design due to distance and angle and that perhaps PL lenses (at 52mm) might give it that extra amount to eliminate the issue.

The PL's are just starting to ship so hopefully shooters start to compare the issue from lens to lens, including any lens mount adapters being used, and EF vs PL.

I could go for a PL mount version of the mini 4.6k if this is the case, as I have a full set of them.

I kinda mean the opposite, i would think that the distance of the EF mount forces the use of lenses where the light is fairly straight out the back, therefore any shading issues should be minimised. I think we'll need to keep an eye on the threads on BMC to see what lenses are being reported and perhaps draw a conclusion from that?

cheers
Paul

 

[starcentral]

Ah gotcha, thanks for the clarity and explanation.

 

[ronnyjay]

Now THAT was some good fire keying.

 

[ronnyjay]

I'm a director - not a technical expert - so excuse me if I'm insulting any of you who already know this. But I've always thought that the slight peripheral blurring was desirable. The Japanese gave it the term "bokeh". Related to circle of diffusion. It directs the eye to the intended location in the frame.

 

[Ben Scott]

I'm a director - not a technical expert - so excuse me if I'm insulting any of you who already know this. But I've always thought that the slight peripheral blurring was desirable. The Japanese gave it the term "bokeh". Related to circle of diffusion. It directs the eye to the intended location in the frame.

This isn't even vaguely related to bokeh.

Bokeh is the nature of the out-of-focus points of light in an image, usually due to the shape and size of a lens' iris. In recent times it has been used to describe the entirety of the out-of-focus part of an image.

Vignetting, which is what this is, is the falloff of LIGHT towards the edge of a frame. It is more closely related to the porthole effect of looking through a toilet roll tube and there being black around the image circle. Typically happens when the image circle of the film gate/sensor is larger than the image circle of the lens. Here it appears the image circle of the lens is larger than the body mount aperture ability to evenly throw light.

That said I wouldn't be surprised to hear some on the BMC forums saying this was somehow a cool attribute and not a huge issue

 

[Nirv]

I'm a director - not a technical expert - so excuse me if I'm insulting any of you who already know this. But I've always thought that the slight peripheral blurring was desirable. The Japanese gave it the term "bokeh". Related to circle of diffusion. It directs the eye to the intended location in the frame.

Circle of Confucius.

 

[starcentral]

Good news?

I know a local guy here in Toronto who received his mini 4.6K earlier this week and I asked him to shoot a white wall, etc.. and he sent me the results yesterday and there was absolutely no magenta corner shadowing in the frame. There was some subtle fall off on the edges but this is pretty normal on really any camera. I will see if he'll let me post it, etc..

We're planning to do some side by side tests with 4.6K and my F55 in the upcoming weeks which should be interesting (and fun).

 

[Ben Scott]

Not really. Poor QC means it is a crapshoot whether you get one that works or not.